Ankle-foot prostheses, also known as transtibial prostheses, are used to help regain walking function in individuals having amputations below the knee. Existing commercial transtibial prostheses generally can be classified into two main types: passive transtibial prostheses and powered transtibial prostheses. As passive transtibial prostheses do not contain actuators, these cannot provide net positive work during a gait cycle, as does an intact human ankle. This lack of positive work hinders amputees from restoring a more natural gait behavior. For some passive prostheses, spring-clutch mechanisms can be used to adjust ankle angle in walking, or improve metabolic walking economy. However, clinical studies showed that due to the inability of passive prostheses to supply net positive work across the stance, transtibial amputees with passive prostheses have various problems during walking, such as non-symmetric gait patterns and higher metabolic cost as compared to the gait of intact individuals.
For powered prostheses, actuators are employed to deliver net positive energy to propel the body forward during walking. This decreases the amputees' effort in walking or metabolic cost significantly and, in addition, allows more normal gaits to be achieved. However, powered prostheses present certain challenges. The two main issues are the use of actuators and power. Although motors are widely used in prostheses. It is difficult to provide the required maximum torque while meeting size and weight requirements. For example, for a 75 kg male person, the ankle-foot complex weighs about 2 kg. During walking, the maximum torque and power would reach 120 Nm and 320 W, respectively. Commercial batteries (e.g., lithium-polymer battery) are usually used for powering prostheses. The battery should provide the energy needed for daily walking. However, with the battery energy density limitation, most commercial batteries cannot supply the energy consumption for an entire day without battery recharge/replacement while meeting the size and weight requirements for an ankle-foot prosthesis.
In some conditions, the batteries cannot supply the enough energy for the powered prostheses due to the excessive walking of the amputees. The powered prostheses will work in passive mode. In that case, we hope the powered prostheses also can reproduce some function of intact ankle, to achieve a relatively normal gait.